1. Field of the Invention
The present invention relates to a method of fabricating a truss core sandwich panel and, more particularly, to an economically feasible method for fabricating lightweight, high strength structural sandwich panel materials which are stabilized by a truss shaped core.
2. Description of the Prior Art
Solid metal plates, made from either steel or aluminium, are common structural members. Plate materials have the advantage of being quite strong, but the disadvantage of being quite heavy.
At the other end of the spectrum is a structural panel of the honeycomb core type. A honeycomb core panel is much more efficient than a plate material in that it can achieve significant buckling strength at a significantly reduced weight. On the other hand, one of the problems with a honeycomb core panel is that it is limited to relatively thin face sheets and it is quite expensive to manufacture.
Between the extremes of plate material and honeycomb core panels is what is commonly referred to as a truss core sandwich panel which is fabricated utilizing two face sheets stabilized by a corrugated sheet or truss core. Such a truss core sandwich panel, while not being quite as efficient in the buckling mode as a conventional honeycomb core panel, is much more efficient than plate material, much lighter than plate material, and can be made with face sheets of significantly heavier gauge than possible with a honeycomb core panel.
The difficulty encountered heretofor with truss core sandwich panels is in the attachment of the second face sheet, which is a difficult and costly fabrication process. That is, once the first face sheet is connected to one side of the corrugated sheet, that side of the corrugated sheet is no longer available for contact in connecting the other side thereof to the second face sheet. As a result, the second face sheet has been secured to the corrugated sheet in a variety of different manners.
For example, this type is truss core sandwich panel is commonly used in structural parts for airplanes where rivets are used. The first face sheet is attached to the corrugated sheet with driven rivets and generally the second face sheet is attached to the core using blind rivets. Rivets are generally satisfactory for heavy gauge materials, although the fabrication process is time consuming, typically because of the necessity to countersink the heads of the rivets to make them flush with the outer surface of the panel for aircraft-type applications. With thin panels, rivets are simply unacceptable.
Brazing and bonding joining techniques can be utilized for complete components but are not adaptable to a panel material for subsequent fabrication into a structure. That is, in either brazing or bonding, a low melting point alloy or glue is used between the corrugated sheet and the second face sheet and the entire panel is subjected to elevated temperatures to cause curing of the connecting material. However, if a panel formed in this manner is to be subsequently machined and welded in a complete component, the welding will cause the bonding or brazing material to deteriorate, thereby contaminating the weld.
Laser welding and electron beam welding are possible techniques for joining the second face sheet to the corrugated sheet. However, the welds are typically so narrow that at least two passes must be made to make a structural connection. Furthermore, electron beam and laser welding equipment is very expensive and takes much more time than coventional resistance welding techniques.
Because of the above difficulties, truss core sandwich panels are used almost exclusively in the fabrication of detailed structural components wherein the face sheets are stabilized by a truss core and joined by resistance welding. These components are relatively small (short in the corrugation direction) and the second face sheet is resistance welded to the truss core using a mechanically expandable mandrel. That is, a multiple-part mandrel is fabricated so that in its collapsed condition, it may be positioned between the first face sheet and the corrugated sheet, where the corrugated sheet contacts the second face sheet. The mandrel is then mechanically expanded to the height of the corrugated core is as to fill the gap between the first face sheet and the corrugated sheet. The first and second face sheets are positioned between opposed welding electrodes, with the expandable mandrel coplanar with the electrodes. By passing a current between the first and second electrodes, the second face sheet may be welded to the corrugated sheet. Then, the mandrel is collapsed mechanically and the steps repeated.
While the above process works, the expandable mandrel is expensive to fabricate and the process is simply too slow, making the finished product overly expensive. While such a slow, expensive process is suitable for a detailed structural component, it would not be suitable for the fabrication of a family of truss core sandwich panels which may be used as a structural panel building material in the manufacture of completed components.
In fabricating a family of truss core sandwich panels for use as a structural panel building material, it is desirable to fabricate the panels in significant lengths, such as up to twelve feet. If resistence welding is the joining process, it is necessary to use a mandrel and to have the mandrel extend between the truss core and the first face sheet connected thereto. Obviously, this can be done with an expandable mandrel, but with the problems discussed previously. If a solid mandrel is used, it would have to be at least as long as the panel, in excess of twelve feet. Initially, it would be extremely expensive to manufacture a copper mandrel this long and the mandrel would be quite heavy. Any slight deformation of the panel due to weld distortion would cause a curvature of the panel, making it virtually impossible to remove the mandrel.
Another possibility would be to slot the mandrel every few inches to provide bending flexibility. While this approach might work well for a few rows of welds, after continued use, the edges of the slots would break down from the electrode wheel pressure, thus causing local welding problems. Also, from continued use in flexing, the mandrels would break at the slots.
Another possible technique for providing the mandrel with bending flexibility would be to reduce the height of the mandrel, by say 0.050", and to add a 0.050" copper strip to the back of the mandrel. In this case, the copper strip is flexible so that after welding, the copper strip may be pulled out and this added mandrel clearance will allow for easy removal of the mandrel, even in a distorted panel. While this system has potential, when considering larger panels and an automated welding machine with multiple electrodes, an excessive amount of electrode copper mandrel material per panel would be required.